1. Field of the Invention
The present invention relates to light valves utilized in display devices, and in particular, to a light valve incorporating a light absorbing thin film stack to prevent penetration of incident light into the underlying silicon substrate.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which, depending upon the image to be projected, either permits or prevents light transmission. Those RGB bands of light which are permitted to be transmitted through the light valves are then combined by dichroic mirrors or a prism. A projection lens then magnifies and projects the image onto a projection screen.
FIG. 1 illustrates a conventional LCD projection-type imaging system 100. Imaging system 100 includes a light source 101. White light is emitted from light source 101. Once the light hits the prism 103, the light is separated into its red, green and blue colored bands of light by dichroic filter coatings. Colored light is directed toward liquid crystal display (LCD) light valves 105. When reflected off light valve 105, the colored light waves travel back through the prism and through projection lens 107. Lens 107 magnifies and projects the synthesized color image onto projection screen 109.
Conventional LCD light valves are formed by confining a thin layer of liquid crystal material between a top plate and a bottom plate. The top plate is a translucent substrate (typically glass) having one large electrode on a surface adjacent to the liquid crystal material. The bottom plate is generally interconnect overlying a storage capacitor structure formed within a silicon substrate.
FIG. 2 illustrates a cross-sectional view of adjacent pixel cell structures lacking a light absorbing layer, that form a portion of a conventional light valve. Portion 200 of the conventional light valve includes a glass top plate 202 bonded to an interconnect 204 by a sealing member (not shown). The sealing member serves to enclose a display area and to separate glass plate 202 from interconnect 204 by a predetermined minute distance. Thus, the light valve has an inner cavity 206 defined by the glass plate 202 and interconnect 204. Liquid crystal material 211, such as polymer dispersed liquid crystal (PDLC), is sealed in inner cavity 206.
Portion 200 of the conventional light valve depicted in FIG. 2 shows adjacent pixel cells 210a and 210b having reflective pixel electrodes 212a and 212b, respectively. Reflective pixel electrodes 212a and 212b are formed as part of third metallization layer 214 of interconnect 204. The surfaces of adjacent pixel electrodes 212a and 212b are covered with a reflecting layer 216. Reflecting layer 216, serves to reflect away white light incident to the pixel cell as described above in connection with FIG. 1. Adjacent pixel electrodes 212a and 212b are electrically coupled to respective storage capacitor structures 218a and 218b formed in underlying silicon substrate 205.
During operation of pixel cells 210a and 210b, driving circuits (not shown) are electrically coupled with storage capacitors 218a and 218b through row select lines 220a and 220b formed as part of first metallization layer 222 of interconnect 204. Storage capacitors 218a and 218b in turn transmit voltages to pixel cell electrodes 212a and 212b through portions of first, second, and third metallization layers 222, 224, and 214 of interconnect 204.
First metallization layer 222 is electronically isolated from silicon substrate 205 by first intermetal dielectric layer 226. Second metallization layer 224 is electronically isolated from first metallization layer 222 by second intermetal dielectric layer 225. Third metallization layer 214 is electronically isolated from second metallization layer 224 by third intermetal dielectric layer 228.
The selective application of voltage to pixel electrodes 212a and 212b switches pixel cells 210a and 210b of light valve 200 on and off. Specifically, a voltage applied to a pixel electrode varies the direction of orientation of the liquid crystal material on the pixel electrode. A change in the direction of orientation of the liquid crystal material at the pixel electrode changes the optical characteristics of the light traveling through the liquid crystal. If the light valve contains twisted nematic crystal, light passes through the light valve unchanged where no voltage is applied to the pixel electrode, and the light is polarized if a voltage is applied to the pixel electrode. If the light valve contains PDLC, light passes through the light valve unchanged where a voltage is applied to the pixel electrode, and light is scattered if no voltage is applied to the pixel electrode.
In the conventional light valve shown in FIG. 2, incident white light can penetrate into interconnect 204 through small gap 230 between adjacent pixel electrodes 212a and 212b. Incident light wave 232 can enter small gap 230, refract at corners 234 of the pixel cell electrodes 212a and 212b, and then reflect off of the second layer of interconnect metallization 224 through a variety of paths until finally penetrating silicon substrate 204.
Penetration of incident light 232 into silicon substrate 204 can induce unwanted currents that can disturb the charge present in storage capacitors 218a and 218b. As a result of this fluctuation in charge, the luminance of pixel cells 210a and 210b may change between succeeding write states, causing the image to xe2x80x9cflicker.xe2x80x9d The flicker produced by the penetrating light waves reduces image quality, and can cause eye strain in an observer.
Existing devices have addressed this problem by incorporating a simple light absorbing layer in the interconnect region. FIG. 3 illustrates a cross-sectional view of adjacent pixel cell structures including a simple light absorbing layer, that form a portion of a conventional light valve. The light valve shown in FIG. 3 is identical to the light valve shown in FIG. 2, except that a simple light absorbing layer 350 has been placed within the second intermetal dielectric layer 328. Simple light absorbing layer is typically composed of a highly optically absorbing material, such as TiN.
FIG. 3 indicates that while most of incident light wave 332 entering narrow gap 330 is absorbed by simple light absorbing layer 350, some incident light is reflected from the surface of light absorbing layer 350. This reflected light can travel through interconnect 304 in a variety of paths before penetrating silicon substrate 305 and giving rise to electrical currents within silicon substrate 305, disturbing charges stored in storage capacitor structures 318a and 318b. 
Therefore, a need exists for a light absorbing layer that not only absorbs incident light, but which also prevents reflection of incident light that could ultimately lead to penetration of light into the underlying silicon substrate of the pixel cell.
The present invention relates to a light absorbing thin film stack which is formed above the silicon substrate of an integrated circuit. This light absorbing thin film stack is designed to block penetration of light into the underlying silicon substrate.
In one embodiment of a light valve in accordance with the present invention, a light absorbing thin film stack is formed within the highest level intermetal dielectric of the interconnect.
The light absorbing thin film stack is formed from a surface layer combination and a backstopping absorbing layer. Each of the layers making up the surface layer combination alters the phase angle of light waves as compared to the immediately preceding layer. The thicknesses of the surface layer combination are tailored to generate destructive interference of reflected light. A thick backstopping absorbing layer positioned behind the surface layer combination ensures that there is no transmission of that portion of the incident light not reflected by the surface layer combination.
The optical characteristics of this first embodiment promotes sufficient absorbance of incident light and destructive interference of reflected light to eliminate penetration of light into the underlying silicon substrate.
Specifically, the films making up the surface layer combination are chosen such that the top layer of the stack has a higher index of refraction than the material lying above it. In turn, the middle layer has a lower refractive index than the top layer. The bottom backstopping layer has a higher index of refraction than the middle layer.
In a first embodiment of a light absorbing thin film stack in accordance with the present invention, the surface layer combination is composed of two layers: a thin (≈100 xc3x85) layer of TiN on top of thicker (≈550 xc3x85) layer of silicon dioxide. The backstopping absorbing layer is composed of a thick (≈1700 xc3x85) layer of TiN.
The composition and thickness of the top and middle layers are tailored to yield substantially different optical thicknesses. The differing optical thicknesses force each of the reflected waves to destructively interfere. This destructive interference attenuates the reflectance of light into the intermetal dielectric.
The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.